Recently, liquid crystal displays are extensively used as display devices of, e.g., portable notebook personal computers. These liquid crystal display devices incorporate a cold-cathode fluorescent lamp as a socalled back light in order to illuminate a liquid crystal display panel from the back. Turning on this cold-cathode fluorescent lamp requires an inverter capable of converting a low DC voltage of a battery or the like into a high AC voltage of 1,000 Vrms or more in an initial lighting state and about 500 Vrms in a steady lighting state. Conventionally, a winding transformer is used as a boosting transformer of this inverter. In recent years, however, a piezoelectric transformer which performs electric conversion via mechanical energy and thereby performs boosting is beginning to be used. This piezoelectric transformer has a generally unpreferable characteristic, i.e., largely changes its boosting ratio in accordance with the magnitude of an output load (load resistance). On the other hand, this dependence upon a load resistance is suited to the characteristics of an inverter power supply for a cold-cathode fluorescent lamp. Accordingly, a piezoelectric transformer has attracted attention as a small-sized, high-voltage power supply meeting the demands for a low profile and a high efficiency of a liquid crystal display. An example of a control circuit for this piezoelectric transformer will be described below with reference to FIG. 1.
FIG. 1 is a block diagram of a piezoelectric transformer control circuit as the prior art.
In FIG. 1, reference numeral 101 denotes a piezoelectric transformer; 102, a load such as a cold-cathode fluorescent lamp connected to the output terminal of the piezoelectric transformer 101; 103, a detecting resistor Rdet for detecting a current flowing in the load; 104, a rectifying circuit for converting an AC voltage generated in the detecting resistor 103 into a DC voltage; 105, an error amplifier for comparing a voltage (to be referred to as a load current detection voltage hereinafter) Vri rectified by the rectifying circuit 104 with a reference voltage Vref and amplifying the difference as a comparison result; 106, a voltage-controlled oscillation circuit for outputting a signal having an oscillation frequency (to be referred to as an oscillation signal hereinafter) in accordance with the output voltage from the error amplifier 105; and 107, a driving circuit for driving the piezoelectric transformer 101 on the basis of the oscillation signal from the voltage-controlled oscillation circuit 106, and an input voltage Vi (DC current).
FIG. 2 is a view showing an example of the internal arrangement of the driving circuit as the prior art.
In FIG. 2, reference numeral 107a denotes a transistor such as a FET (Field Effect Transistor) for generating an AC voltage by switching the input voltage Vi in accordance with an oscillation signal from the voltage-controlled oscillation circuit 106; and 107b, a winding transformer for applying the AC voltage to the piezoelectric transformer 101. Since the winding transformer 107b has a filter effect by a secondary inductive component and a capacitive component of the piezoelectric transformer 101, a rectangular-wave voltage generated by switching of the transistor 107a is changed into a sine wave on the secondary side of the winding transformer 107b to be applied to the piezoelectric transformer. This sine-wave voltage drives the piezoelectric transformer 101 to generate a high AC voltage at the output terminal of the piezoelectric transformer 101.
The operation of the control circuit with the above arrangement will be described below with reference to FIGS. 3A and 3B.
FIGS. 3A and 3B are graphs for explaining an example of frequency characteristics for an output voltage from the piezoelectric transformer and a load current.
As shown in FIG. 3A, the piezoelectric transformer 101 has a hilly resonance frequency characteristic whose peak is the resonance frequency of the piezoelectric transformer 101. It is generally known that a current flowing in the load 102 due to the output voltage from the piezoelectric transformer 101 also has a similar hilly characteristic. In FIG. 3B, this load current is represented by the load current detection voltage Vri (characteristic curve A). Control using a right-side (falling) portion in this characteristic will be described below. When the power supply of this control circuit is turned on, the voltage-controlled oscillation circuit 106 starts oscillating at an initial frequency fa. Since no current flows in the load 102 at that time, the voltage generated in the detecting resistor 103 is zero. Accordingly, the error amplifier 105 outputs a negative voltage, as a result of comparison of the load current detection voltage Vri with the reference voltage Vref, to the voltage-controlled oscillation circuit 106. In accordance with this voltage, the voltage-controlled oscillation circuit 106 shifts the oscillation frequency of an oscillation signal to a lower frequency. Therefore, as the frequency is shifted to a lower frequency, the output voltage from the piezoelectric transformer 101 rises, and the load current (load current detection voltage Vri) also increases. When the load current (load current detection voltage Vri) and the reference voltage Vref become equal to each other, the frequency stabilizes (fb). In the control circuit which operates in this manner, even if the resonance frequency of the piezoelectric transformer 101 changes due to a temperature change or a change with time, the oscillation frequency of the voltage-controlled oscillation circuit 106 shifts in response to the change to always hold the load current substantially constant.
In the control circuit shown in FIG. 1, therefore, frequency control is so performed that the load current detection voltage Vri becomes equal to the reference voltage Vref, and the load current is held at a predetermined value by this frequency control.
However, if the input voltage Vi increases in the piezoelectric transformer control circuit as the prior art, a voltage for driving the piezoelectric transformer 101 increases to raise an output voltage from the piezoelectric transformer 101 (characteristic curve B in FIG. 3B). Since the rise in output voltage increases a current in the load connected to the output terminal of the piezoelectric transformer 101, the load current detection voltage Vri becomes higher than the reference voltage Vref, resulting in a shift of the frequency of the oscillation signal to a higher frequency fc. To the contrary, if the input voltage falls, the load current decreases to shift the frequency of the oscillation signal to a lower frequency. In general, the input/output conversion efficiency of the piezoelectric transformer is the highest when the piezoelectric transformer is driven at a frequency around the resonance frequency of the piezoelectric transformer, and lowers as the frequency shifts to a higher frequency. Therefore, although the control circuit has a desirable function of holding the load current at a predetermined value even if the input voltage Vi changes, the driving frequency of the piezoelectric transformer varies owing to variations in input voltage Vi, resulting in a low conversion efficiency.